So far, a high strength steel sheet has been used a lot in applications such as bolts, PC steel wires and line pipes, and, it is known that when the tensile strength becomes 980 MPa or more, due to intrusion of hydrogen into steel, the hydrogen embrittlement (such as the pickling embrittlement, plating embrittlement and delayed fracture) is caused. Compared with this, since a steel sheet thickness is thin, when hydrogen is intruded, hydrogen is released in a short time. Additionally, from the view point of the workability and weldability, since a steel sheet of 780 MPa or more has not been used so much, an aggressive countermeasure to the so-called hydrogen embrittlement has not been considered.
However, recently, from the necessity of attaining light weight in automobiles and of improving the collision safety thereof, there has been a rapidly increasing tendency in applying the press molding or bending work to a ultrahigh-strength steel sheet of 980 MPa or more to use in a reinforcement material such as bumpers or impact beams or a sheet rail. Furthermore, also parts such as pillars to which the press molding or bending work are applied are demanded to be high in the mechanical strength. Accompanying this, a demand for an ultrahigh-strength thin steel sheet provided with the hydrogen embrittlement susceptibility resistance is becoming high.
As a steel sheet responding to such the demand, in particular, a steel sheet that uses TRIP (TRansformation Induced Plasticity) steel is gathering attention.
The TRIP steel is a steel sheet where an austenite texture remains and, when the working deformation is applied, due to the stress, residual austenite (residual γ) is induced to transform to martensite to enable to obtain large elongation. As the kinds thereof, some may be cited. Examples thereof include a TRIP type composite texture steel (TPF steel) that contains residual austenite with polygonal ferrite as a matrix phase; a TRIP type tempered martensite steel (TAM steel) that contains residual austenite with tempered martensite as a matrix phase; and TRIP type bainitic steel (TBF steel) that contains residual austenite with bainitic ferrite as a matrix phase. Among these, the TBF steel has long been known (described in, for example, non-patent document 1), and has such advantages as that, due to hard bainitic ferrite, high strength is readily obtained, and, in the texture, fine residual austenite grains are easily formed in the boundary of lath-shaped bainitic ferrite and such the texture transformation shows very excellent elongation. Furthermore, the TBF steel also has such an advantage from the production point of view as that it can be easily manufactured by a single heat treatment process (continuous annealing process or plating process).
When the hydrogen embrittlement resistance (hydrogen embrittlement resistance properties) of the TRIP steel are improved, it is considered to convert the technology relating to bar steel and bolt steel where various kinds of elements are added to a steel. For instance, in non-patent document 2, it is reported that, when in a metallographic texture formed mainly of tempered martensite, additive elements such as Cr, Mo and V that show the resistance to temper softening are added, the delayed fracture resistance is effectively improved. This is a technology where alloy carbide is precipitated in a steel to utilize as a hydrogen trap site and thereby the delayed fracture form is converted from the intergranular fracture to the transgranular fracture. Furthermore, in patent document 1, it is reported that an oxide mainly made of Ti and Mg effectively inhibits the hydrogen-related defect from occurring. Furthermore, in patent document 2, it is reported that when a dispersion state of oxide and sulfide of Mg, composite precipitated or precipitated compound is controlled and residual austenite in a microstructure of a steel sheet and the mechanical strength of the steel sheet are controlled, the elongation (ductility) and the delayed fracture resistance after the working are made compatible.    Patent document 1: JP-A-11-293383    Patent document 2: JP-A-2003-166035    Non-patent document 1: NISSIN STEEL TECHNICAL REPORT, No. 43, December 1980, pp 1-10    Non-patent document 2: “New Development in Elucidation of Delayed Fracture (Okurehakaikaimei no shintenkai)” (published by The Iron and Steel Institute of Japan in January, 1997, pp 111-120)